Heat-sensitive recording materials comprising a colorless or pale-colored leuco dye and a color developer which, when heated, combines with the leuco dye to form a color have been widely used for various purposes. Common heat-sensitive recording materials can be prepared by dispersing and pulverizing a leuco dye and a color developer such as a phenolic substance each separately by a wet mill into micron-size particles, mixing together the particles of the two components, adding auxiliaries such as a binder, sensitizer, filler, lubricant, stabilizer, dispersant, antifoamer and the like to give a liquid coating composition, and applying the coating composition to a substrate such as papers, films or synthetic papers. The foregoing heat-sensitive recording materials form colored record images on chemical reaction which occurs when at least one of the leuco dye and the color developer is fused to come into contact with the other. With the advantage of easily producing record images, especially sharp images, this type of heat-sensitive recording materials are widely used as recording media for facsimiles, printers, etc.
As information media have been recently diversified, the heat-sensitive recording materials have found wider applications, e.g. as ticket papers useful for machines for automatically producing tickets such as commutation tickets, as bar code papers in a POS (point of sales) system or as labels for commercial goods or the like. The heat-sensitive recording materials used for such new applications have become more frequently exposed to severer conditions than when used under circumstances heretofore known. For example, the heat-sensitive recording materials cause fading at a recorded portion or the formation of color at an unrecorded portion on contact with a commutation-ticket holder comprising vinyl chloride and dibutyl phthalate or like plasticizer, becoming less valuable as recording materials, which means that the heat-sensitive recording materials are required to have resistance to plasticizers. For use as labels for foods, the heat-sensitive recording materials need to have resistance to water, alcohols, oils and acetic acids and the like in addition to resistance to plasticizers.
To meet such performance requirements, various methods for preventing the penetration of water, chemicals or the like have been employed by coating the heat-sensitive color-forming layer on the substrate with an overcoating agent such as an aqueous solution of a water-soluble resin or an emulsion of a water-insoluble resin or the like (e.g. a styrene-maleate copolymer as disclosed in Japanese Unexamined Patent Publication No.62-280073, a polyvinyl alcohol-vinyl acetate emulsion as disclosed in Japanese Unexamined Patent Publication No. 61-229590, an aqueous solution of a polyvinyl alcohol-acrylamide copolymer as disclosed in Japanese Unexamined Patent Publication No.62-278086, etc.). These methods can satisfy said performance requirements to some extent but can never exhibit concurrently a satisfactory resistance to water, plasticizers and chemicals as well as a sufficient resistance to sticking and the desired color forming property.